Network Working Group                                          J. Fischl
Internet-Draft                               CounterPath Solutions, Inc.
Intended status: Standards Track                           H. Tschofenig
Expires: January 10, 2008
                                                            July 9, 2007

  Session Description Protocol (SDP) Indicators for Datagram Transport
                         Layer Security (DTLS)

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This specification defines how to use the Session Description
   Protocol (SDP) to signal that media will be transported over Datagram
   Transport Layer Security (DTLS) or where the SRTP security context is
   established using DTLS and.  It reuses the syntax and semantics for
   an SDP 'fingerprint' attribute that identifies the certificate which
   will be presented during the DTLS handshake.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  DTLS Certificates . . . . . . . . . . . . . . . . . . . . . . . 3
   4.  SDP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Session Description for RTP/AVP over DTLS . . . . . . . . . . . 4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 5
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     9.2.  Informational References  . . . . . . . . . . . . . . . . . 7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 7
   Intellectual Property and Copyright Statements  . . . . . . . . . . 8

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1.  Introduction

   Session Description Protocol (SDP) RFC 2327 [7] has been used to set
   up the transport of various types of media with RTP [9] over UDP
   [10], TCP [14], and TLS [2].  DTLS [12] is a protocol for applying
   TLS security to datagram protocols such as UDP and DCCP [1].  This
   specification defines new SDP protocol syntax that allow SDP to
   indicate that DTLS should be used to transport the media when TLS is

   The handling of TLS sessions in SDP is defined in [2] that discusses
   only TLS over TCP.  This document extends that specification to also
   deal with TLS over datagram protocols such as UDP and DCCP and when
   (D)TLS is used to establish keys for SRTP as in [5]

   [[NOTE: This document has a major dependency on work currently going
   on in the MMUSIC WG to mechanisms for SDP capability negotiation
   which will enable this sort of best-effort encryption.  When that
   work is finished, this draft will be harmonized with it.
   Furthermore, the contents of this document will be integrated into

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [6].

3.  DTLS Certificates

   The two endpoints in the exchange present their identities as part of
   the DTLS handshake procedure using certificates.  This document uses
   certificates in the same style as described in Comedia over TLS in
   SDP [2].

   If self-signed certificates are used, the content of the
   subjectAltName attribute inside the certificate MAY use the uniform
   resource identifier (URI) of the user.  This is useful for debugging
   purposes only and is not required to bind the certificate to one of
   the communication endpoints.  The integrity of the certificate is
   ensured through the fingerprint attribute in the SDP.  The
   subjectAltName is not an important component of the certificate

   If the endpoint is also able to make anonymous sessions, a distinct,
   unique, self-signed certificate SHOULD be provided for this purpose.

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   The generation of public/private key pairs is relatively expensive.
   Endpoints are not required to generate certificates for each session.

   The endpoints MAY cache their certificates and reuse them across
   multiple sessions.

   [Editor's Note: Certificate lifetime issues will be discussed in a
   future draft version.]

4.  SDP

   In addition to the usual contents of an SDP [13] message, each 'm'
   line will also contain several attributes as specified in RFC 4145
   [11] and [2].

   The endpoint MUST use the setup and connection attributes defined in
   "TCP-Based Media Transport in the SDP" [11].  For the purposes of
   this specification, a setup:active endpoint will act as a DTLS client
   and a setup:passive endpoint will act as a DTLS server.  The
   connection attribute indicates whether or not to reuse an existing
   DTLS association.

   A certificate fingerprint is the output of a one-way hash function
   computed over the distinguished encoding rules (DER) form of the
   certificate.  The endpoint MUST use the certificate fingerprint
   attribute as specified in [2].

   TODO: The MMUSIC working group is currently studying the problem of
   signalling in SDP the ability/desire to initiate a secure channel
   rather than an insecure one [3][4].  We need to use those techniques
   when they are finalized.

5.  Session Description for RTP/AVP over DTLS

   This specification defines new tokens to describe the protocol used
   in SDP "m=" lines.  The new values defined for the proto field are:
   o  When a RTP/AVP stream is transported over DTLS with DCCP, then the
      token SHALL be DCCP/TLS/RTP/AVP.
   o  When a RTP/AVP stream is transported over DTLS with UDP, the token
   o  When a RTP/AVP stream is transported over TLS with TCP, the token
   o  When media is transported over DTLS with UDP, the token SHALL be

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   o  When media is transported over DTLS with DCCP, the token SHALL be

   For RTP profiles other than AVP, a new token should be defined in the
   form of DCCP/TLS/RTP/xyz, UDP/TLS/RTP/xyz and TCP/TLS/RTP/xyz where
   xyz is replaced with an appropriate token for that profile.

6.  IANA Considerations

   This specification updates the "Session Description Protocol (SDP)
   Parameters" registry as defined in Appendix B of RFC 2327 [7].
   Specifically it adds the following values to the table for the
   "proto" field.

   Type            SDP Name                     Reference
   ----            ------------------           ---------
   proto           TCP/TLS/RTP/AVP              [RFC-XXXX]
                   UDP/TLS/RTP/AVP              [RFC-XXXX]
                   DCCP/TLS/RTP/AVP             [RFC-XXXX]
                   UDP/TLS                      [RFC-XXXX]
                   DCCP/TLS                     [RFC-XXXX]

   Note to RFC Editor: Please replace RFC-XXXX with the RFC number of
   this specification.

7.  Security Considerations

   When using self signed certificates, the signalling protocol used to
   transport the SDP MUST ensure the integrity of the SDP so that the
   fingerprint attribute can not be altered.  Failure to do this would
   allow a attacker to insert themselves in the media channel as a man-
   in-the-middle.  A method of ensuring the integrity of the SDP when
   transporting over the SIP RFC 3261 [8] signalling protocol is
   described in [15]

8.  Acknowledgments

   Cullen Jennings contributed substantial text and comments to this
   document.  This document benefitted from discussions with Francois
   Audet, Nagendra Modadugu, Eric Rescorla, and Dan Wing.  Thanks also
   for useful comments by Flemming Andreasen, Rohan Mahy, David McGrew,
   and David Oran.

9.  References

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9.1.  Normative References

   [1]   Kohler, E., "Datagram Congestion Control Protocol (DCCP)",
         draft-ietf-dccp-spec-13 (work in progress), December 2005.

   [2]   Lennox, J., "Connection-Oriented Media Transport over the
         Transport Layer Security (TLS)  Protocol in the Session
         Description Protocol (SDP)", draft-ietf-mmusic-comedia-tls-06
         (work in progress), March 2006.

   [3]   Andreasen, F., "SDP Capability Negotiation",
         draft-ietf-mmusic-sdp-capability-negotiation-05 (work in
         progress), March 2007.

   [4]   Andreasen, F., "SDP Capability Negotiation: Requirements and
         Review of Existing Work",
         draft-ietf-mmusic-sdp-capability-negotiation-reqts-01 (work in
         progress), March 2007.

   [5]   McGrew, D. and E. Rescorla, "Datagram Transport Layer Security
         (DTLS) Extension to Establish Keys for  Secure Real-time
         Transport Protocol (SRTP)", draft-ietf-avt-dtls-srtp-00 (work
         in progress), July 2007.

   [6]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

   [7]   Handley, M. and V. Jacobson, "SDP: Session Description
         Protocol", RFC 2327, April 1998.

   [8]   Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
         Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
         Session Initiation Protocol", RFC 3261, June 2002.

   [9]   Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
         "RTP: A Transport Protocol for Real-Time Applications", STD 64,
         RFC 3550, July 2003.

   [10]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
         Conferences with Minimal Control", STD 65, RFC 3551, July 2003.

   [11]  Yon, D. and G. Camarillo, "TCP-Based Media Transport in the
         Session Description Protocol (SDP)", RFC 4145, September 2005.

   [12]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
         Security", RFC 4347, April 2006.

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9.2.  Informational References

   [13]  Handley, M., "SDP: Session Description Protocol",
         draft-ietf-mmusic-sdp-new-26 (work in progress), January 2006.

   [14]  Lazzaro, J., "Framing RTP and RTCP Packets over Connection-
         Oriented Transport", draft-ietf-avt-rtp-framing-contrans-06
         (work in progress), September 2005.

   [15]  Fischl, J., Tschofenig, H., and E. Rescorla, "Session
         Initiation Protocol (SIP) for Media Over Transport Layer
         Security (TLS)", June 2006.

Authors' Addresses

   Jason Fischl
   CounterPath Solutions, Inc.
   Suite 300, One Bentall Centre, 505 Burrard Street
   Vancouver, BC  V7X 1M3

   Phone: +1 604 320-3340

   Hannes Tschofenig


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